In many types of aircraft, particularly fixed wing aircraft, it is standard practice to avoid flying, under aerodynamically generated lift, at velocities close to stall airspeeds. This practice is even more strictly adhered to in the case for unmanned air vehicles (UAV), and improves safety and minimizes risk of loss or damage to the air vehicles, which can occur when losing aerodynamic control thereof.
However, this practice also results in a restriction in the operation of the air vehicle, particularly UAV's, to above a specified airspeed (which includes a speed safety margin imposed on stall airspeed), reducing operation in an otherwise allowable part of the flight envelope. The limitation is especially relevant for the flight and take-off/landing phases of small and medium size UAV flying at reduced airspeeds in windy air, and is also applicable to other aircraft. For such cases, reducing the specified airspeed (which may be accompanied by an increased angle of attack to maximize lift) eventually leads to stall of the wings and loss of aileron's aerodynamically generated controlling forces, and may produce uncontrollable dynamic response of the aircraft, leading to the development of spin modes that are difficult to recover from, particularly in the case of unmanned air vehicles. In particular, there is often a fast variation in aerodynamic characteristics of the aircraft at stall and post-stall angles of attack that are highly unstable/unreliable and are difficult to predict, and which render the aircraft (specially UAV's) difficult to control in flight. The actual speed safety margin (to avoid the stall flight regime) imposed on aircraft operation is often dependent on the stall pattern of the wing and may vary, depending on the specific case and required flight conditions, and the standard speed safety margin adopted for UAV operations is: Vflight≧1.2 Vstall.
Stall characteristics for subsonic-type wings or aerofoils (synonymously referred to also as airfoils) may be classified as follows:                Abrupt stall is characterized by the fast drop of the lift at stall angle of attack, typically resulting in an approximate 20-50% loss of lift coefficient within about 1° to about 3° after the stall angle of attack, depending on the level of maximum lift (e.g., moderate or high lift wings). This type of stall is associated with flow separation at the leading edge of the wing (leading edge stall), or, with very fast progress of trailing edge separation        Moderate stall is characterized by the gradual development of trailing edge separation at the stall of the wing and moderate loss of the lift at post-stall angles of attack, typically resulting in a rate of loss of lift coefficient at post-stall angles of attack that is approximately close to the rate or slope of lift coefficient gain prior to the stall angle of attack, but of opposite slope thereto. This type of stall is associated with moderate progress of trailing edge separation at post-stall angles of attack.        Mild stall is characterized by almost constant level of the lift at post-stall domain and is associated with slowly creeping trailing edge separation that moderates the rate of lift losses at high angles of attack, typically resulting in an approximately constant lift coefficient (within between about 5% and about 10% of the maximum lift coefficient for at least about 5° after the stall angle of attack).        
The stall angle of attack may be defined as the angle of attack at which maximum lift coefficient (or up to about 99% of maximum lift coefficient) is first realized. The stalling speed is dependent on the weight (W) of the air vehicle, maximum lift coefficient (Cl max), wing area (Sw), and air density (ρ), and is generally defined asVstall=(2W/(ρ*Cl max*Sw))0.5 
Stall characteristics also tend to deteriorate as maximum lift is increased, resulting in more difficulties when attempting to comply with considerations of flight safety and to avoid unfavorable stall patterns. This is especially relevant for high-lift, long endurance wings of some UAV, such as for example the HERON high-lift long endurance UAV, manufactured by IAI, Israel. In the HERON, optimum endurance performance is via high loitering lift coefficients, which requires high maximum lift.
FIGS. 1(a) and 1(b) illustrates some variations of aircraft endurance factor (CL1.5/CD) for the different levels of configuration parasite drag, and marks a general region therein generally relevant for UAV with high and moderate aspect ratio wings. For some cases of interest (for example, where there is increased parasitic drag due to external installations, protruding payloads, engine-airframe integration, installation of cooling systems, etc.), there is a clear advantage of high-lift, loitering flight. However, with the need to impose a speed safety margin on the minimum loiter velocity Vloiter=1.2Vstall, the full potential for increased lift that may be generated when flying at lower velocities may not be achieved for such vehicles, particularly UAV's.
Advanced aerofoil design concepts and available CFD tools facilitate development of wings for many new aerodynamic configurations. This is especially relevant for unmanned air vehicles, where variety of mission requirements, multi-point design specifications and imposed non-aerodynamic constraints often render existing aerofoils impractical for project evaluation/development work, and the design of new customized wings is typically a development choice. The cases of small and medium size UAV (the class of Tactical UAV, having a weight of between about 50 kg and about 500 kg) may be of particular interest for development of high-lift UAV wings because of the encountered range of relatively low Reynolds numbers. For unswept wings of conventional wing-body-tail configurations, the typical chord Reynolds numbers close to stall airspeeds are around Re=0.3-1.0*106. This is above the Reynolds numbers of Mini UAV wings, with their characteristics strongly dominated by viscous flow effects, but still belonging to domain of low Reynolds numbers aerodynamics.
Aerofoil configurations with enhanced lift-carrying capabilities can be attractive for development of UAV wings because of their potential to provide an answer to many desirable UAV characteristics, and the concept of high-lift, loitering flight has been adopted for the development of subsonic and transonic long endurance UAV (Refs. 5, 6, 7, 10). This approach is especially relevant for design of configurations with high aspect ratio wings and increased level of parasitic drag, in which optimum endurance performance of UAV may be realized at high loitering lift coefficients, leading to very demanding requirements for maximum lift. Schematic illustration of the concept is presented in FIGS. 1(a) and 1(b), using simplified parabolic approximation of aircraft drag polars, and it may be appreciated that for low level of parasite drag (the case of sailplanes), moderate values of loitering lift coefficients (CL loiter˜0.8-1.0) and maximum lift (CL max˜1.3-1.5) are sufficient for realization of the best endurance performance. Conversely, for increased values of aircraft parasite drag, the region of optimum endurance performance tends to deviate to domain of high loitering lift coefficients. Two-element aerofoils (referred to herein as “SA aerofoils”), with high maximum lift, were used in the development of the HERON, a long endurance UAV produced by IAI, Israel, and demonstrated in flight its enhanced loitering performance (Refs. 11, 12). Other high-lift, two-element aerofoils for mission-adaptive wing sections have been adopted at IAI as a main design concept for aerodynamic development of long endurance UAV (Refs. 5, 7, 10), and these aerofoils have a built-in option of take-off/landing flaps, ailerons, airbrake and decambering at maximum speed flight—for example, a two-element, slotted aerofoil (denoted SA-19), was developed in mid 90's for the experimental IAI Firebird UAV.
SA-aerofoils are designed straightforward for deployed flap position and for high loitering lift coefficient. They rely on rotation of the second element around external hinge point for adjustment of aerofoil performance to different flight regimes. Furthermore, SA-aerofoils of the prior art are inherently high lift wing sections that rely on positive deflections of the second element for a further enhancement of maximum lift, which comes with a certain degradation of stall pattern that is typical for the wing sections employing high-lift devices, so that standard speed safety margin Vflight≧1.2Vstall is usually applied to operation of such wing sections.
For the development of small and medium size UAV with moderate aspect ratio wings, high-lift, loitering flight retains its attraction as a design concept because of a further increase of parasitic drag which may be attributed to protruding payloads, drag-consuming installations, power plant/airframe integration, engine cooling, etc.